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Just bought a 6a with O-360 engine, and CHTs get very got (>450F by the time I rotate)

N

nickw9815

Member
Hey folks,

I just bought an RV6a and the CHTs are very hot. Engine is an O-360 carbureted. Are these following numbers normal?

These are the numbers if I do a cold start, and then taxi for <5 minutes to the run-up area and take off. If I do a hot start (such as a quick fuel stop), these numbers are ~25-50 degrees higher

Airspeed & RPMCHTsOAT
Idle - 800 rpm~350 F80 F
At 100ft AGL after takeoff, 90 knots, 2500 rpm~450 F80 F
Enroute climb at 110kts, 2300rpm, 500fpm420-450 F~70 F
Cruise, 130kts IAS, 2200rpm, 10,500' MSL~390 - 400 F (I am used to seeing these temps when at full power, not 60% power)~60 F

The builder who I bought it from never does hot starts, so he did not have a problem with this. He essentially would do a very quick taxi, take off, then immediately reduce power in the climb. Even with a cold start, I am unable to climb more than 500fpm without having CHT's go above 450 (and would prefer to keep them under 400). When doing a hot start, I essentially have to cut power as soon as I rotate and climb at ~200fpm to keep the CHTs < 450.

The engines have these little metal plates in front. I heard that we could remove these or trim these to be smaller?
 

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We have slick magnetos. We just had the ignition timing looked at as part of the pre-buy, and it seemed to be good. Would using automotive or aviation spark plugs make a difference to CHTs is the timing was the same?
 
If you have engine monitoring, share your fuel flow on departure. Also share carburetor model ( data plate on carburetor) . Constant speed or fixed pitchprop? RPM at WOT takeoff roll. SUREFLY or Slick ignition?
 
nickw9815 said:
We have slick magnetos. We just had the ignition timing looked at as part of the pre-buy, and it seemed to be good. Would using automotive or aviation spark plugs make a difference to CHTs is the timing was the same?
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Without a special harness , you cant use auto plugs on slick
 
With the tiny bit I can see from the picture, the baffles look “normal”, so assuming there is no gross modification or omission we can’t see, then ignition timing is the first place to look
 
agree with the others. Need more info. Need all CHTs, EGT, FF, Oil temp, etc.

but as a general statement, no, these temps you are reporting are not “normal”. Yours are higher than typically seen for the operations you describe.
 
Mags are slick mags. I am not sure if spark plugs are aviation or automotive. How do you tell the difference?
Carburetor is from Precision Airmotive. Don't know the exact year, but a new one was installed in 1998 so it should be around then.

On take off, Cylinder 1 is the hottest by ~30 degrees. In cruise, cylinder 1,2 are the same, #3 is + 20 F, #4 is +10F. I don't have a fuel flow gauge. EGTs are high 1400's.

Adding a photo of the panel here during a climb for more context. In this photo we leveled off at ~7500 to let things cool, then began climb again. At higher altitudes with cooler air we can add more power but still not at full throttle. Also, the mixture looks super lean in the photo, but it is rigged to be very rich. So full rich is ~1/2 travel.
 

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nickw9815 said:
Mags are slick mags. I am not sure if spark plugs are aviation or automotive. How do you tell the difference?
Carburetor is from Precision Airmotive. Don't know the exact year, but a new one was installed in 1998 so it should be around then.

On take off, Cylinder 1 is the hottest by ~30 degrees. In cruise, cylinder 1,2 are the same, #3 is + 20 F, #4 is +10F. I don't have a fuel flow gauge. EGTs are high 1400's.

Adding a photo of the panel here during a climb for more context. In this photo we leveled off at ~7500 to let things cool, then began climb again. At higher altitudes with cooler air we can add more power but still not at full throttle. Also, the mixture looks super lean in the photo, but it is rigged to be very rich. So full rich is ~1/2 travel.
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What part number carb? If you are full rich, high 1400s on the EGTs seems lean.. I would think takeoff mixture should be about 1280-1330 or so.. I agree that checking mag timing would be in order, something isn’t right.
 
Taltruda said:
What part number carb? If you are full rich, high 1400s on the EGTs seems lean.. I would think takeoff mixture should be about 1280-1330 or so.. I agree that checking mag timing would be in order, something isn’t right.
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Yep, lean.

If full rich then a carb problem or intake air leaks, or both.

Carl
 
I went through some of this when I got my plane, and I'll try to summarize what everyone above is saying (outside of the auto plugs thing as I don't know the consequences of that). They're asking about ignition timing because if it's past 25 BTDC, then your CHTs will be high. You can even knock this down to 21-23 if you're having a really hard time and this should help your CHTs (at the cost of a small amount of power).

Next thing people are talking about is your carb, because there is a model that allows better fuel flow than the other (even the low FF model can be retrofitted with a kit for higher fuel flow). You seem to be running lean with those EGTs even though they're relative, so you probably don't have enough fuel going to your cylinders. Fuel in the cylinders, particularly at high power settings, helps a lot with cooling. If your fuel flow is low, which I'm guessing it is, you're going to run hot.

Final thing is the baffling. I didn't look at yours, but if it's not forming a nice seal on your cowling, you're losing cooling air to places that don't need it, causing your CHTs to be high.

One final thing I did that got me a good bit of cooling was tape off half of my cabin air inlet behind cylinder #3. This 2" hole is unnecessarily big, and taping off half of it both increased the efficiency of the cabin heat and gave me 10-20 degrees of cooling across all cylinders.

Good luck, it's a pain in the butt chasing cooling issues but you'll get to the bottom of it with diligence. Your temps are very high. Most would say that is really bad, some would say people overreact with CHTs, up to you to decide how important it is to you.
 
Maybe it is an illusion bit the lean lever seems to be back a bit. Did you lean a lot after climb? Are you FULL RICH on takeoff? Does that lever make a complete throw at the carburetor?

Just more to think about. Good luck!
 
nickw9815 said:
Mags are slick mags. I am not sure if spark plugs are aviation or automotive. How do you tell the difference?
Carburetor is from Precision Airmotive. Don't know the exact year, but a new one was installed in 1998 so it should be around then.

On take off, Cylinder 1 is the hottest by ~30 degrees. In cruise, cylinder 1,2 are the same, #3 is + 20 F, #4 is +10F. I don't have a fuel flow gauge. EGTs are high 1400's.

Adding a photo of the panel here during a climb for more context. In this photo we leveled off at ~7500 to let things cool, then began climb again. At higher altitudes with cooler air we can add more power but still not at full throttle. Also, the mixture looks super lean in the photo, but it is rigged to be very rich. So full rich is ~1/2 travel.
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Im not 100% sure, but a chance you have a fuel injection system vs a traditional updraft float carburetor . Its not a lot posted on VAF about the product. Slick mags, high egts early in flight, looks to me to be a too little fuel flow, causing high CHTemps. No fuel flow measurements hampers not knowing for sure. Id call manufacture( if a PA fuel injection system) for how to increase flow at WOT & FULL RICH. You can easily lower flow with mixture when in cruise.
 
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nickw9815 said:
Hey folks,

I just bought an RV6a and the CHTs are very hot. Engine is an O-360 carbureted. Are these following numbers normal?

These are the numbers if I do a cold start, and then taxi for <5 minutes to the run-up area and take off. If I do a hot start (such as a quick fuel stop), these numbers are ~25-50 degrees higher

Airspeed & RPMCHTsOAT
Idle - 800 rpm~350 F80 F
At 100ft AGL after takeoff, 90 knots, 2500 rpm~450 F80 F
Enroute climb at 110kts, 2300rpm, 500fpm420-450 F~70 F
Cruise, 130kts IAS, 2200rpm, 10,500' MSL~390 - 400 F (I am used to seeing these temps when at full power, not 60% power)~60 F

The builder who I bought it from never does hot starts, so he did not have a problem with this. He essentially would do a very quick taxi, take off, then immediately reduce power in the climb. Even with a cold start, I am unable to climb more than 500fpm without having CHT's go above 450 (and would prefer to keep them under 400). When doing a hot start, I essentially have to cut power as soon as I rotate and climb at ~200fpm to keep the CHTs < 450.

The engines have these little metal plates in front. I heard that we could remove these or trim these to be smaller?
Click to expand...


I had this same problem when i bought a 6. the carb on the plane was too lean. check the data plate on the carb.. i bet you have a 3878 and what you need is a 4164. i tried baffles, timing, cowl flaps.. then i changed the carb and all the high temp probelms went away.. check your carb first ~!!!!!!!
 
I’m NOT a fuel injection guy, but thought I understood that for FI systems fuel pressure is much higher than a carburetor based system. You panel picture shows what I expect for a carb .5 to 5psi vs 40+ psi for fuel
Injection systems. Also panel has Fuel pump switch and fuel boost pump switch. Fuel pump switch is showing OFF. Just things seeming odd/different to me and my limited knowledge if indeed you have a FI system upgrade to an O320. If 26 years old like you mentioned, maybe it needs servicing if not happening in past. Good luck.
 
nickw9815 said:
Also, the mixture looks super lean in the photo, but it is rigged to be very rich…
Click to expand...
Probably a terminology thing here, but the rigging and resulting mixture is defined by the mechanical stops and adjustments at the carb. Where the cockpit lever or knob ends up is a “result” of rigging. Point being, full rich at half travel does not make it “more rich” at any travel beyond that. And on that point, if you CAN move the knob/lever more, then you have a problem.

All the above to say that it is beginning to look like you have a mixture control issue. Might be time to start from scratch and re baseline. -adding bandaids on top of an uncorrected root cause will only make things worse in the long run.
 
It's hard to tell from the one pic of the engine, but it looks like your baffle seals on the back baffle don't curl around slightly to the front. They should fold over and seal against the top cowling as shown in those instructional videos [BTW, those Kitplanes videos are really good and cover many subjects]. When you have CHT issues across the board, it's likely one of three things: a mag timing, baffle leak, or fuel delivery problem. But others have already stated this. The good news is that those are easy to check and not too expensive to fix.

There was a recent VAF post where someone was having similar issues. Somebody made a suggestion to remove the lower half of the cowling and have a helper shine a light all around the flexible seals to see if you can see light (air leaks) while looking inside the inlets. Do that light trick on the inter-cylinder baffles as well, as that's a common place for air leaks--especially where the metal should seal up against the cylinder bases. Even a few small leaks can have a big effect on cooling.

Lastly, Mike Busch has a webinar on CHTs titled "How Hot is Too Hot". It's worth an hour of your time to watch that on YouTube. And while many people will say the temps you're experiencing are acceptable, I side with Mike and prefer them to be a bit lower. My car engine's redline is around 6,000 RPM, but I rarely let it ever get close to that limit for the same reason. But that's just me.
 
The builder who I bought it from never does hot starts, so he did not have a problem with this. He essentially would do a very quick taxi, take off, then immediately reduce power in the climb. Even with a cold start, I am unable to climb more than 500fpm without having CHT's go above 450 (and would prefer to keep them under 400). When doing a hot start, I essentially have to cut power as soon as I rotate and climb at ~200fpm to keep the CHTs < 450.
Click to expand...


I am curious, did you take this RV-6 up for a test flight with the seller and was this condition present at that time?
Folks go to great length to get pre buy inspections and not go for an actual flight to see and confirm the condition of the airplane and engine.

I am sure with all the advice you've received, you'll be able to correct this condition.
I certainly would not want to have to reduce power as soon as I rotate to keep the Cylinders from melting.
I would have passed om the plane until the condition was rectified, what you describe is borderline unsafe.
 
nickw9815 said:
The engines have these little metal plates in front. I heard that we could remove these or trim these to be smaller?
Click to expand...

If the #1 cylinder is running hot compared to #3, then some trimming of the plate in front of #1 is in order.
However, If I read correctly, #1 is hot initially, and then #3 is hotter later / in cruise / etc.
I don't think this is the place to start.

[Same logic with #2 and #4]

LOTS of good advice above. I'll just add your EGTs (at full rich???) are higher than I'd expect.
  1. When was the carb last overhauled. Probably a long time ago. Over 12 years?
  2. Does full rich on the knob push the carb arm to the stop?
  3. Model number of the carb?
  4. Good chance there are air leaks
  5. Air temp was what? I see you are in "California". Huge difference between Palm Springs and Half Moon Bay :)
 
Captain obvious here, but I would run a scope down the tube over the oil cooler and make sure it did not suck a rag or bird covering the oil cooler. Make sure you can see through it.
Larry
 
Just an FYI, my baffle seals are not great and due to be replaced. I can see areas of obvious blow by on the top cowl. Still, it hasn’t been a pressing issue as I can easily manage cooling in all aspects of flight.
Your baffle seals would have to be really bad to see the numbers you’re posting.
Look elsewhere first, then fix the seals if they show blow by.
 
350 CHT with less than 5 minutes taxi and run-up at 80 OAT? Seems like timing to me. Too lean or baffle seal shouldn't cause 350F during taxi.

The only other thing I would check is the accuracy of the CHT probes...test them in boiling distilled water...corrected for current atmospheric pressure.
 
I didn’t see anyone comment on the two terms “cold start” & “hot start” used in the original post. I normally use these terms to describe the procedures used to start either a cold engine, or re-start a previously run hot engine.
Does “cold start” mean the builder used to jump in the plane, fire up & take off as quick as possible before the engine had time to warm up?
Does “hot start” mean he wouldn’t even be tempted to take off if the engine was previously run???

For your sake, I hope this engine doesn’t have many hours of operation with this setup.
 
Couple things to note...

  1. your oil temp of ~235 is definitely on the high side
  2. Altering the tabs on the front won't make a difference. They are not designed to block cooling air, only redirect it to the back cylinders
 
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What do we know about the engine? Was it a factory new engine? Total time, stock, overhauled, recently topped?
 
I had a friend that bought a Cessna 182 with a recently installed factory new engine. He complained that it ran way to hot. So we took of the top cowl and discovered all of the flexible baffle seals were flexed out away from the engine, where the baffle touches the inside of the cowl: by reforming the seals so that they flexed toward the engine, the temperatures were returned to normal levels.
 
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